(a) Field of the Invention
The present invention relates to an LCD (liquid crystal display) device and, more particularly, to an in-plane switching mode LCD device which uses a lateral electric field for rotating LC (liquid crystal) molecules in an LC layer.
(b) Description of the Related Art
An LCD device includes a pair of substrates sandwiching therebetween an LC layer, the pair of substrates including a TFT (thin-film-transistor) substrate and a counter substrate (color filter substrate). Each substrate includes thereon an alignment film for aligning the initial direction of the LC molecules in the LC layer. For achieving a higher contrast ratio in the LCD device, it is desired to obtain a uniform surface of the substrate by reducing the step difference thereon, and a uniform rubbing treatment for the alignment film to thereby uniformly align the direction of the LC molecules. Although there are some techniques for improving the contrast ratio other than the reduction of the step difference, the factors restricting the contrast ratio will be described first.
FIG. 11 shows the TFT substrate of a conventional LCD device in a top plan view, and FIG. 12 is a sectional view thereof taken along line XII—XII in FIG. 11. On a transparent substrate 1, there is provided a chrome film constituting a plurality of scanning lines (gate lines) 2 and a plurality of common lines 3 extending parallel to one another. The scanning lines 2 and the common lines 3 are covered by a gate insulation film 5, on which a semiconductor (silicon) layer 6 and a plurality of drain lines 7 are consecutively formed. The drain lines 7 supply therethrough pixel signals
Drain electrodes 8, which constitute part of the drain lines 7, and the source lines 9 are connected to the semiconductor layer 6. The source electrode 9 constitutes a storage electrode 10 in an area above the common line 3, and is extends toward the central area of the pixel to constitute a lower pixel electrode 11. The semiconductor layer 6, drain electrode 8, source electrode 9, and a part of the scanning line 2 below the semiconductor layer 6 form a TFT (thin film transistor) used as a switching element in the LCD device.
A protective film 12 and an interlayer dielectric film 13 cover the switching element including the gate insulation film 5. An upper pixel electrode 14 and a common electrode 15 are formed on the interlayer dielectric film 13 for applying a lateral electric filed to the LC layer.
A portion of the common electrode 15 extending above the drain line 7 in an area sandwiched between adjacent common lines 3 constitutes a shield common electrode 16. The shield common electrode 16 shields the leakage electric field leaking out from the drain line 7 in the area between adjacent common lines 3 toward the effective pixel area of the pixel upon application of the electric field. The upper pixel electrode 14, common electrode 15 and shield common electrode 16 are formed in a common layer, which opposes the scanning line 2 and drain line 7 with an intervention of the insulation films including the protective film 12 and interlayer dielectric film 13. This common layer is disposed on the top surface of the insulation films near the LC layer to thereby constitute a topmost layer among the interconnect layers, and is made of a transparent material such as ITO (indium-tinoxide).
The upper pixel electrode 14 is electrically connected to the lower pixel electrode 11 via a contact plug 17 penetrating the insulation films in the vertical direction. On the other hand, the common electrode 15 and the shield common electrode 16 are electrically connected to the underlying common line 3 via a contact plug 18 penetrating the insulation films in the vertical direction as well.
The shield common electrode 16 is disposed to overlap the underlying drain line 7, and has a width larger than the width of the drain line 7, thereby protruding from each side edge of the drain line 7 by a protruding length “L”. The protruding length “L” is generally 4 μm or more, and preferably 6 to 8 μm in the case where especially smaller crosstalk is desired, for an effective suppression of crosstalk between the drain line 7 and the pixels by preventing the leakage electric field from the drain line 7 toward the upper pixel electrode 14. This crosstalk may be referred to as a vertical crosstalk because the crosstalk occurs between the pixels arranged in the vertical direction on the screen.
The alignment film (not shown) covers the effective pixel area (or opening area), overlying the upper pixel electrode 14, common electrode 15 and shield common electrode 16 on the interlayer dielectric film 13. The alignment film is subjected to a rubbing treatment before completing the TFT substrate 100, which is disposed to oppose a counter substrate or color filter substrate 300 with an intervention of the LC layer 200.
A structure of the LCD device for shielding the leakage electric field leaking out from the drain line is described in JP-A-1998-186407, for example. The LCD device described therein includes a pair of shield common electrodes (common lines) sandwiching therebetween a signal line (drain line) in the thickness direction of the films, thereby achieving a reduction of the crosstalk due to the leakage electric field substantially without reducing the effective area ratio. The pair of common lines are connected together via contact plugs arranged along the signal line for further shielding the leakage electric field from the signal line. The effective area ratio means a ratio of the effective pixel area to the total pixel area, the effective pixel area passing therethrough light for display of an image.
In the structure as described in the patent publication, the common lines, which are connected together via the contact plugs arranged along the signal line and maintained at a common potential, may effectively prevent the leakage electric field from the signal line. However, the common lines having a larger capacitance between the same and the signal line causes a signal distortion due to the large capacitive load of the signal line, thereby involving a larger in-plane brightness difference, especially in a large-screen LCD device. The term “in-plane brightness difference” means a difference in the brightness between the pixel at the top row and the pixel at the bottom row, which are applied with the same signal voltage at the common signal terminal of the LCD device. Thus, it is difficult to use the structure in the commercial LCD devices. In addition, the contact plugs also reduce the effective area ratio of the pixel.
The reason of the reduction of the contrast ratio caused by the protrusion of the shield common lines, as described before, will now be described in more detail, with reference to FIGS. 12 and 13. FIG. 13 shows the electric field generated in the conventional LCD device having the structure of FIG. 12.
As shown in FIG. 12, the light “B” transmitted from the backlight device in the area (area-B) corresponding to the protrusion length “L” advances from the transparent substrate 1, via the gate insulation film 5, protective insulation film 12, interlayer dielectric film 13, shield common electrode (ITO electrode) 16, alignment film (not shown), LC layer 200, and alignment film (not shown), toward the color filter substrate 300. On the other hand, in the area (area-C) between the shield common electrode 16 and the upper pixel electrode 14, the light “C” advances from the transparent substrate 1 via the gate insulation film 5, protective insulation film 12, interlayer dielectric film 13, alignment film (not shown), LC layer 200 and alignment film (not shown), toward the color filter substrate 300. The area-B corresponding to the protrusion of the shield common electrode or length “L” is herein referred to as quasi-effective pixel area, the area-C corresponding to the gap between the shield common electrode 16 and the upper pixel electrode 14 is herein referred to as an effective pixel area, and the area (area-A) which completely blocks the light due to the upper pixel electrode 14 is herein referred to as a masked area,
As to the “black brightness” of each area, the area-A has a black brightness of zero because the light is substantially completely blocked therein, and the difference in the black brightness between the area-B and the area-C is substantially zero because the ITO scarcely blocks the light.
As to the “white brightness” of each area, the situation is different from the black brightness. It is to be noted that an electric field is applied between the pixel electrode and the common electrode, whereby the LC molecules in the LC layer 200 having a refractive index anisotropy rotate in the plane parallel to the substrates. Thus, the light passed by the polarization plate (not shown) on the TFT substrate 100 is changed in the polarization direction thereof due to the birefringence phenomenon to thereby pass through the polarization plate on the color filter (CF) substrate 300. FIG. 13 qualitatively and schematically depicts the electric field applied on the LC layer 200 in the LCD device.
In the area-C, the larger horizontal component of the electric field rotates the LC molecules in the LC layer 200 in a larger amount, whereby a large amount of light passes through the polarization plate to display a white color. On the other hand, in the area-B, the electric field or line of electric force is directed in the oblique direction, which causes a smaller horizontal component of the electric field, whereby the LC molecules in the area-B rotates in a less amount compared to those in the area-C. Thus, the amount of light passing through the polarization plate on the CF substrate side is smaller, and white brightness in this area is lower. In the area-A, the drain line 7 substantially completely blocks the light, whereby the white brightness is zero in this area.
Here, assuming that:
the black brightness, white brightness and area ratio in the area-A are a (Candela), A (Candela) and X %;
the black brightness, white brightness and area ratio in the area-B are b (Candela, B (Candela) and Y %; and
the black brightness, white brightness and area ratio in the areas are c (Candela, C (Candela) and Z %, where X+Y+Z=100,
the contrast ratio CR of whole the LCD device, which is defined as the ratio of the average white brightness to the average black brightness, can be expressed by the following formula:
                    CR        =                                                            AX                100                            +                              BY                100                            +                              CZ                100                                                                    aX                100                            +                              bY                100                            +                              cZ                100                                              =                                                    AX                +                BY                +                CZ                                            aX                +                bY                +                cZ                                      =                                                            BY                  +                  CZ                                                  bY                  +                  cZ                                            .                                                          (        1        )            It is to be noted here that “a” and “A” are substantially zero. Since “b” is substantially equal to “c”, as described above, the formula (1) can be simplified as follows:
                    CR        =                                            BY              +              CZ                                      c              ⁡                              (                                  Y                  +                  Z                                )                                              .                                    (        2        )            If it is assumed that the ratio (W0) of the sum of the effective area (area-C) and the quasieffective area (area-B) to the total area including these areas and the masked area (area-A) is a constant, then the contrast ratio CR is expressed as follows:
  CR  =                    BY        +        CZ                    c        ×                  W          0                      =                            BY          +                      C            ⁡                          (                                                W                  0                                -                Y                            )                                                cW          0                    =                        W          1                -                              W            2                    ⁢          Y                    where W1 and W2 are constants.
More specifically, in the structure shown in FIG. 13, a larger area for the area-B, i.e., a larger area ratio “Y” for the area-B, reduces the contrast ratio. FIG. 14 shows the relationships between the protrusion length “L” and the crosstalk and between the protrusion length “L” and the measured value for the contrast ratio. As understood from this figure, a larger length for the protrusion length “L” allows a larger amount of the shield effect to thereby reduce the crosstalk, whereas the larger protrusion length reduces the contrast ratio.
FIG. 15 shows a structure wherein the LCD has a larger contrast ratio by removing the area-B. In this structure, the shield common electrode 116 is made of chrome instead of the ITO used in FIGS. 12 and 13, thereby forming a masked area (area-A) for blocking the light “B” in this area. This structure allows the contrast ratio to increase up to 857 in a theoretical value.
However, the shield common electrode 116 made of chrome involves a problem in that the shield common electrode 116 may be peeled-off from the interlayer dielectric film 13 made of an organic substance. This is because there is a poor adhesion in the boundary between a metal and the organic film. In addition, a chrome film has a larger stress therein such as 1 GPa, as compared to 0.2 to 0.6 GPa of the stress in the ITO, the larger stress being enhanced by a higher temperature used in the deposition of the chrome film. The larger stress increases the possibility of the peel-off from the organic film,
The higher deposition temperature also causes oxidation of the surface of the chrome film, which may degrade the reliability of the electric connection of the same with the anisotropic conductive film (ACF). For improvement of the reliability in the electric connection, an ITO film may be formed on the chrome film, which increases the number of fabrication steps, however. Due to the reasons as described above, it is not practical to replace the ITO shield common electrode by a metallic shield common electrode such as made of chrome.
Due to the reasons as described heretofore with respect to the conventional LCD device, the common shield electrode 16 covers the drain line 7 and generally has a relatively large protrusion protruding from each side edge of the drain line 7 to thereby reduce the crosstalk between the drain line 7 and the pixel electrode 14.
Although the crosstalk may be reduced by reducing the length of the protrusion as viewed in the widthwise direction of the drain line, this causes a poor function of the protrusion for shielding the electric field.